Ignoring the actual hand for a second (I'm not familiar with GIB but this looks like a value bid, not a LAW-based bid, compatible with smerriman's explanation), Cohen introduces the "bid to the level of your total number of trumps" as a shortcut for "bid 3 over 2 with 16 total trumps, but not 3 over 3". The idea is that, conditional on us having an 8 card fit, the opponents are unlikely to have a 10 card fit or longer (if they have a 10 card fit that's 10 out of their 26 cards spoken for, and the other 15 have to break 5-5-6 to give us 'only' an 8 card fit, in which event we even have a double fit). Conversely, it is possible but unlikely the opponents only have a 7 card fit (our other 18 cards have to split 6-6-6). So with exactly an 8 card fit, assume the number of total trumps is 16 or 17, and act accordingly. Similar logic can be applied to a 9 card fit, but it gets less accurate with 10 card fits or longer. To the best of my knowledge the statement of the LAW is that the total number of tricks (defined as the number of tricks NS can make in their optimal trump suit plus the number EW can take in their optimal trump suit - neither of which need be the longest trump suit) is (often) equal to the number of trumps (longest trump suit of NS + longest trump suit of EW). All rules on bidding X over Y, or bidding to level Z, are derived from this in combination with bridge score tables and some statistics, along with adjustments to make the law more accurate.